What exactly does the term small signal mean?
In most transistor amplifier circuits, you can think of a small signal as the difference between the actual level of a signal and its quiescent state or, in other words, the a.c. component of a signal. In many cases the term small signal can be a bit of a misleading one. Sometimes the small signals in a circuit can be as big as the quiescent values (if not bigger). For example, the collector voltage of a common-emitter amplifier can vary by several volts. However, in order to maintain linearity, it is vital that the variation in base-emitter voltage of a transistor is kept small (typically no more than a few millivolts, depending on the circuit). Any larger a.c. values of vbe and the circuit is operating in a large signal sense. The approximately linear relationship between vbe and ic is no longer valid and so tools like mutual conductance etc. cannot be applied.
